1. Field of the Invention
The present invention relates to a bidirectional direct current-to-alternate current (DC/AC) inverter, and in particular to a drive control method for a bidirectional DC/AC inverter at the time of charging a battery.
2. Description of the Related Art
FIG. 1 is a diagram showing a conventional bidirectional DC/AC inverter.
The bidirectional DC/AC inverter 30 shown by FIG. 1 comprises a filter 34 constituted by coils 31 and 32 and by a capacitor 33; a bridge circuit 39 constituted by four switching elements 35 through 38 and connected to the filter 34; a bridge circuit 45 constituted by four switching elements 41 through 44 and connected to the bridge circuit 39 by way of a capacitor 40; a bridge circuit 51 constituted by four switching elements 47 through 50 and connected to the bridge circuit 45 by way of a transformer 46; and a capacitor 53 and a coil 54 which are equipped between the bridge circuit 51 and a battery 52. Note that the switching elements 35 through 38, switching elements 41 through 44, and switching elements 47 through 50, are insulated gate bipolar transistors (IGBT) for example, with each of them being connected to a diode in parallel.
The above noted bidirectional DC/AC inverter 30 makes the switching elements 41 and 44 and switching elements 42 and 43 turn on and off alternatively in the case of charging the battery 52. That is, an alternate current (AC) power externally input to the bridge circuit 39 by way of the filter 34 is rectified by the diodes which are parallelly connected to the switching elements 35 through 38, respectively, of the bridge circuit 39 and also smoothed by the capacitor 40, thereby being converted into a direct current (DC) power in the case of charging the battery 52. Then the DC power is converted into an AC power by the bridge circuit 45 and is output to the bridge circuit 51 by way of the transformer 46. Then the AC power is rectified by the diodes which are parallelly connected to the switching elements 47 through 50, respectively, of the bridge circuit 51 and also is smoothed by the capacitor 53, thereby being converted into a DC power. Then the DC power is supplied to the battery 52 by way of the coil 54.
On the other hand, the above noted bidirectional DC/AC inverter 30 makes the switching elements 47 and 50 and switching elements 48 and 49 of the bridge circuit 51 turn on and off alternatively, and also makes the switching elements 35 and 38 and switching elements 36 and 37 of the bridge circuit 39 turn on and off alternatively in the case of externally outputting an AC power. That is, a DC power obtained from the battery 52 is converted into an AC power by the bridge circuit 51 and is output to the bridge circuit 45 by way of the transformer 46 in the case of outputting an AC power from the bidirectional DC/AC inverter 30. Then the AC power is rectified by the diodes parallelly connected to the switching elements 41 through 44, respectively, of the bridge circuit 45 and also is smoothed by the capacitor 40, thereby being converted into a DC power. Then, the DC power is converted into an AC power by the bridge circuit 39 and is output by way of the filter 34.
As described above, the above noted bidirectional DC/AC inverter 30 drives only the bridge circuit 45 at the time of charging the battery 52, while it drives only the bridge circuits 39 and 51 at the time of outputting an AC power (refer to a patent document 1 for example).
[Patent document 1] Laid-Open Japanese Patent Application Publication No. 2001-37226
Incidentally, a charging method for the battery 52 includes one for charging it while adjusting a charging voltage by controlling a current output from the bridge circuit 45 by making a duty of each the switching elements 41 through 44 of the bridge circuit 45 variable while maintaining an input voltage to the bridge circuit 45 approximately constant, for example. This method is applied for maintaining an input voltage to the battery 52 constantly a little higher than a terminal voltage of the battery 52 in the case of charging the battery 52 by a constant current for example. When increasing the input voltage to the battery 52 with the terminal voltage of the climbing terminal voltage of the battery 52, the duty of each of the switching elements 41 through 44 increases with the climbing terminal voltage of the battery 52.
However, in the method for charging the battery 52 by varying the input voltage thereto as described above, a current output from the bridge circuit 45 becomes small, resulting in decreased efficiency of a DC/AC power conversion of the bridge circuit 45 when the terminal voltage of the battery 52 is low and the duty of each of the switching elements 41 through 44 is accordingly small. Consequently, it is necessary to configure the bridge circuit 45 by using high performance switching elements which are capable of running a larger volume of current when the duty is small, in order to increase the DC/AC power conversion efficiency of the bridge circuit 45 by the application of the above described method, thus facing with a problem of a cost increase, et cetera.